This invention relates to a control for an engine and more particularly to an improved feedback control system and method for a direct injected internal combustion engine.
In the interest of improving engine performance and particularly fuel efficiency and exhaust emission control, many types of engines now employ fuel injection and/or feedback control of the fuel supply to the engine. Fuel injection has the advantages of permitting the amount of fuel delivered for each cycle of the engine to be adjusted. In addition, by utilizing fuel injection and a feedback control system, it is possible to maintain the desired fuel air ratio under a wide variety of engine running condition.
These features are particularly useful with two cycle engines, although not specifically limited thereto. The utilization of these features with two cycle engines is important, however, because of the emission characteristics of the two cycle engine is somewhat poorer than those of the four cycle engine. This is the result of the fact that the engine fires every revolution and there is a fairly substantial overlap during the scavenging and exhaust cycles.
Generally, these systems operate with a combustion condition sensor such as an oxygen (O.sub.2) sensor that outputs a signal indicating whether the mixture is lean or rich. If the mixture deviates from the desired mixture, then incremental adjustments may be made to bring the mixture back into the desired relationship.
When feedback control is utilized with direct cylinder injected engines, the fuel injector is positioned in fairly close proximity to the combustion condition sensor. This is because the combustion condition sensor normally is positioned at a place in or near the cylinder where it will be able to sense the condition of the combustion products immediately toward the end of the combustion cycle.
With two cycle engines, this may be done by placing the combustion chamber so that it communicates with a cylinder at a point adjacent the exhaust port and slightly above it. In this way, the sensor will sense the combustion conditions before the exhaust port opens, but at the time when combustion will be fairly well completed. However, this places the sensor in a position where fuel can also impinge upon it and this can give rise to certain problems.
In addition to the fuel impingement problem, when the combustion condition sensor is positioned in close proximity to the fuel injector, there may be conditions when only minute adjustments in fuel supply are necessary to maintain the desired running condition. However, because of the inherent overshoot nature of feedback control systems, this can cause fairly substantial fluctuations in the air fuel ratio that may be detected by the operator and which may give the operator an uneasy feeling.
The problem is particularly acute in connection with marine applications because the exhaust gases may be discharged to the atmosphere through the body of water in which the watercraft is operating for silencing purposes. The water level can change due to boat motion, changes in both velocity and other factors. In addition, the trim angle and height of the propulsion unit may be altered, and this also will change the back pressure and, accordingly, the desired fuel air fuel ratio.
It is, therefore, a principle object of this invention to provide an improved control strategy for a feedback control engine and feedback control system wherein areas of low fluctuation in air fuel ratio are accommodated by a means other than by adjusting the amount of fuel injected.
It is a further object of this invention to provide an improved control strategy and method for feedback control engines wherein hunting is minimized, particularly during areas where only small adjustments in fuel supply are required.